1. The Field of the Invention
The present invention relates to systems and methods for displaying images on a display device. More specifically, the present information relates to systems and methods for displaying text characters on a display device having separately controllable pixel sub-components, with selected edges of the characters being positioned at high contrast pixel sub-component boundaries.
2. The Prior State of the Art
A key aspect of many technologies, especially computers, is the requirement of visual interaction with an end user. In fact, the data and information displayed by many applications are especially designed to have a visually pleasing and aesthetic appearance. Word processors, for example, typically attempt to emulate an actual piece of paper by causing the screen to display a graphical a piece of paper. As the word processor creates a document, the text and images are placed on the graphic piece of paper as they will appear when physically rendered. The visual interaction of the end user with the display device is therefore an integral part of many technologies.
An important aspect of the visual interaction with an end user is the ability to display color. A typical display device has a screen that has a large number of pixels, and each pixel usually has red, green, and blue sub-pixels. Because the pixels are relatively small, the color discerned by the human eye is a blend of the red, green, and blue lights of each pixel. By varying the intensity of the red, green, and blue lights, a display device is potentially capable of displaying millions of different colors.
One common display device used to display color is a cathode ray tube (CRT) display device. The pixels of a CRT display are arranged in a particular geometry, with each pixel consisting of three phosphors that emit light when excited by a beam of electrons. For CRT display devices, the luminous intensity values assigned to the phosphors are calculated together and the three phosphors in a pixel are controlled together to generate at the pixel a color perceived by the user as having a selected hue, intensity and saturation.
Another common display device, which is typically used for portable computers, is a liquid crystal display (LCD). LCD displays are preferable to CRT displays in many instances primarily because LCD displays are usually smaller, weigh less, and consume less power than comparably sized CRT displays. In computers that rely on batteries for their power, this is a significant advantage. The technology of LCD displays enables them to rival CRT displays in terms of sharpness and resolution.
Several differences, however, exist between CRT and LCD displays. Pixels of LCD display devices, unlike those of CRT display devices, consist of a plurality of pixel sub-components (usually three) that are separately addressable and inherently separately controllable. In addition, the pixels in an LCD display are usually arranged to form horizontal or vertical stripes of same-colored pixel sub-components, whereas the geometry of a the phosphors in a pixel of a CRT display is frequently triangular. In most LCD display devices used with portable computers, the pixels sub-components are arranged vertically, which results in red, green, and blue vertical stripes of same-colored pixel sub-components. Other LCD display devices have pixel sub-components arranged to form red, green and blue horizontal stripes of same-colored pixel sub-components. Other geometries may be used, but vertical and horizontal arrangements are the most common.
The ability of an LCD display or other display to provide high resolution is dependent in part on the size and quantity of the pixels in the LCD display, and in many situations, the resolution of LCD displays is not sufficient to enable text characters to be smoothly drawn or rendered on the LCD display. The limited resolution of the LCD display may have a significant visual impact on the user as the text or image data is rendered on an LCD or other display.
Text characters or fonts, in particular, present unique problems. Technology permits a font to be stored on a computer with high resolution and when a high resolution character is rendered on a low resolution or coarse display device, the character is fitted to the pixel grid of the display device. This results in a loss of information relating to the characters and frequently compromises the shape of the characters, which is contrary to the intent of the typographer.
More specifically, when a character is rendered to a low resolution display device or pixel grid, certain parts of the character as it is stored electronically may not fall exactly on the pixel boundaries As a result, the shape of the character is forcibly changed to conform with pixel boundaries in the pixel grid. The ultimate effect on the character is that the character may be displaced in a certain direction or the stems, serifs, and other features of the character may be somewhat thicker or thinner than originally designed by the typographer.
Conventional text rendering processes that result in text characters being displayed on display devices were originally designed to conform to the CRT model of pixels having three phosphors that are controlled together to display a single color and to represent a single portion of the image. With the advent of large numbers of portable personal computers, existing text rendering processes designed for CRT display devices were simply directly applied to LCD display devices. Such conventional text rendering process, when applied to LCD display devices, use each pixel to represent a single portion of the image, and do not take advantage of the separately addressable nature of the pixel sub-components.
In view of the foregoing, there is a need in the art for techniques for rendering text on LCD display devices that can improve the resolution of the text. It would be desirable to provide systems and methods that could enhance the readability of text and reduce the character distortion that has previously resulted from edges of characters being repositioned to boundaries of full pixels of LCD display devices.
The present invention relates to systems and methods of rendering text and other images on LCD display devices or other display devices having pixels with separately controllable pixel sub-components. According to the invention, individual pixel sub-components represent different portions of a text character or another image, rather than the entire pixel representing a single portion. This is accomplished by mapping spatially different sets of one or more samples of the image data to individual pixel sub-components. Because the pixel sub-components are separately addressable and controllable, the LCD display device operated according to the invention renders images with improved resolution compared to the resolution generated by conventional rendering processes.
While the principles of the invention can be used to render any image, the invention is described herein primarily in the context of text characters. As part of the invention, character data is hinted, or fitted to a grid, such that selected edges of the character fall on high contrast boundaries between pixel sub-components, thereby further improving the appearance and readability of the text. Thus, not only are portions of the character displayed with sub-pixel resolution, but selected edges of the character are also positioned to reduce the color fringing errors or effects that would otherwise be experienced.
The hinting process of the invention involves analyzing the topology of the outline of the character at runtime to identify edges of the character that are to be repositioned to high contract boundaries between pixel sub-components. While the invention extends to a direct and full analysis of the topology of the character, it is often more computationally efficient to analyze portions of the character definition in a font file to identify control points on the character that are to be fitted to the high contrast boundaries and distances between the control points.
Additional advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.